fix regression in tcl execute procedure ("execute 0 prog" blocking if prog closes stdout/stderr file descriptors)

src/xschem.tcl

@@ -43,12 +43,12 @@ proc execute_fileevent {id} {
   append execute(data,$id) [read $execute(pipe,$id) 1024]
   if {[eof $execute(pipe,$id)]} {
       fileevent $execute(pipe,$id) readable ""
+      if { $execute(status,$id) } {
         # setting pipe to blocking before closing allows to see if pipeline failed
         # do not ask status for processes that close stdout/stderr, as eof might
         # occur before process ends and following close blocks until process terminates.
         fconfigure $execute(pipe,$id) -blocking 1
         set status 0
-
         if { [ info tclversion]  > 8.4} {
           set catch_return [eval catch [list {close $execute(pipe,$id)} err options] ]
         } else {
@@ -78,7 +78,14 @@ proc execute_fileevent {id} {
             viewdata "Completed: $execute(cmd,$id)\ndata:\n$execute(data,$id)" ro
           }
         }
-      if { [info exists execute(callback,$id)] } { eval $execute(callback,$id); unset execute(callback,$id) } 
+      } else {
+        # nonblocking close always succeed 
+        close $execute(pipe,$id)
+      }
+      if {[info exists execute(callback,$id)]} {
+        eval $execute(callback,$id)
+        unset execute(callback,$id)
+      } 
       unset execute(pipe,$id)
       unset execute(data,$id)
       unset execute(status,$id)
@@ -117,7 +124,8 @@ proc execute {status args} {
   if { [catch {open "|$args" r} err] } {
     puts stderr "Proc execute error: $err"
     if { [info exists has_x]} {
-        tk_messageBox -message  "Can not execute '$args': ensure it is available on the system. Error: $err" \
+        tk_messageBox -message  \
+          "Can not execute '$args': ensure it is available on the system. Error: $err" \
            -icon error -parent [xschem get topwindow] -type ok
     }
     return -1